“Extrusion” is a method of pressing against a starting billet placed in a hollow cylindrical container by a stem provided at a main crosshead, so as to push it through a die having a desired opening shape arranged close to one end part of the container at the extrusion side and thereby shape it into a bar material, shaped material, tube material, or various other cross-sectional shapes. The main crosshead is driven by a hydraulic main ram provided at a main housing. The die is provided at an end platen side. The end platen is connected by tie rods to an immovable main housing.
During the extrusion process, the container has to be pressed against the die by a certain fixed pressure. This is referred to as a “container seal” operation. Further, after the end of the extrusion process, to cut off and remove the discard, the container is made to retract and clearance between the container and die is secured so as to enable a shear blade to be moved up and down. This is referred to as a “container strip” operation.
A type of operation of an extrusion apparatus operating by a composite system of an electric motor and hydraulic pressure in such an extrusion process is disclosed in PLT 1. In this prior art, a billet holder (container) and ram crosshead (main crosshead) are electrically driven to move until the billet abuts against the die, then hydraulic force is made to act on a pressing ram (main ram) to start the extrusion by an extrusion plunger (stem). The billet holder is sealed against the tool set (die) by traction rods. After the end of extrusion, the pressure of the pressing ram is released, then the billet holder and ram crosshead are electrically driven to return to their retraction limits. The billet holder and ram crosshead are guided in movement by a plurality of pressing support parts.
In this prior art, the extrusion ram has a built-in clamp device, while the traction rods have built-in ring-shaped cylinder clamp units. The working force in this prior art is structured relying on the frictional force obtained by making the parts inside the clamps broader in a tapered manner. In this structure, the sliding parts inevitably become worn. Maintenance is frequently required for replacing parts. However, since the parts are formed inside the clamps, a long time required for replacement and other problems are envisioned. Further, a prefill valve is also built into the back part of the main ram. It is comprised of complicated parts, so time is required for maintenance for replacement of the parts.